1. Field of the Invention
The present invention relates to a method of producing a magnetic carrier that is used in developing methods in which a toner image is formed on an electrostatic latent image bearing member by the development using a two-component developer of an electrostatic latent image formed on the electrostatic latent image bearing member. The present invention further relates to a magnetic carrier produced using this production method.
2. Description of the Related Art
In order to satisfy market needs such as the accelerated color shift in office use, the more intense colorfulness levels sought by the graphics market, and faster speeds for light-duty printing, an even higher image quality, higher stability, and higher durability have in recent years been required from a performance standpoint of the two-component developers used in electrophotographic printing methods.
At the present time, the magnetic carrier present in two-component developers mainly takes the form of a magnetic carrier in which a coat layer is formed by a resin composition on the surface of a ferrite core or a resin core having a magnetic body dispersed therein (these cores are collectively referred to below as magnetic carrier cores).
This resin composition coat layer functions to inhibit charge injection from the magnetic carrier into the photosensitive member, in order thereby to stabilize the charge distribution on the toner and improve the durability in terms of enabling stable charging even during extended use.
Accordingly, it is very important that the resin composition coat layer be uniformly coated on the magnetic carrier core surface.
In the present invention, a uniform coating treatment refers to a state in which the coat layer of the resin composition coats the whole magnetic carrier core surface and in which, in addition to the smoothness of the magnetic carrier surface, the resin density within the coat layer is uniform. This uniform resin density within the coat layer is a state in which openings in the coat layer are not present or, if they are present, they are uniformly dispersed.
Wet coating treatment methods, infra, have frequently been used to execute a uniform coating treatment of the magnetic carrier core surface with the resin composition.
This wet coating treatment refers to methods in which the surface of magnetic carrier core suspended in a fluid bed is spray coated with a coating solution in which the resin composition is dissolved in a solvent, and to methods in which the coating treatment is carried out by immersing the magnetic carrier core in a coating solution in which the resin composition is dissolved in a solvent.
These wet coating treatment methods, because the coating treatment is carried out in a solution, offer the advantage of carrying out a coating treatment by the resin composition on the magnetic carrier core surface that is uniform within the coat layer.
However, a problem with wet coating treatments has been the facile agglomeration of the magnetic carrier when the solvent is evaporated off.
However, when a magnetic carrier that has undergone agglomeration is deagglomerated by stirring, the surface of the magnetic carrier core may be exposed to some degree at the parting surfaces, and as a result the coat layer on the magnetic carrier surface becomes nonuniform and a leakage event, which is the previously mentioned charge leakage event from the magnetic carrier to the photosensitive member, can readily occur.
When such a leakage event occurs, the surface potential of the photosensitive member converges to the developing bias and the development contrast cannot be securely maintained and a blank dot image may be produced.
In addition, this exposure of the magnetic carrier core surface can also prevent the toner charge from being maintained, particularly at high temperatures and high humidities, and a low toner charge after long-term standing can also readily result in, for example, image defects such as fogging.
Moreover, a special drying step is required in order to completely remove the solvent, which causes the takt time to increase, and thus much also remains to be improved from a production standpoint with wet coating treatments. In addition, when the resin composition particles have a weight-average molecular weight Mw of at least 100,000 for the tetrahydrofuran (THF)-soluble matter in the resin component present in the resin composition, dissolution in a solvent is then difficult to achieve and there may be limitations on the selection of the resin composition.
Dry coating treatments, which carry out the coating treatment thermally without using a solvent and using particles of the resin composition, have therefore been introduced as methods that overcome the aforementioned problems associated with wet coating treatments.
For example, the following method is disclosed in Japanese Patent Application Laid-open No. 2011-075855.
First, 2.0 mass parts of resin composition particles (glass-transition temperature (Tg)=98° C./number-average primary particle diameter=0.1 μm) and 100.0 mass parts of a magnetic carrier core are mixed.
Then, coating with the resin is carried out by stirring for 20 minutes using a high-speed stirring and mixing device at a temperature of 90° C. and at 13 m/sec for the rotational velocity in the horizontal direction; the internal temperature is adjusted to 120° C.; and the magnetic carrier is obtained by subsequently carrying out a heat treatment for 30 minutes while stirring at 5 m/sec.
In this method, the entire apparatus is heated by the flow of a thermal medium in a jacket disposed on the inside of the main casing and the temperature of the processed material as a whole is brought to at least the glass-transition temperature (Tg) of the resin composition particles present in the processed material.
However, since in this method the temperature of the entire apparatus is brought to at least the glass-transition temperature (Tg) of the resin composition particles, agglomeration of the magnetic carrier is facilitated, and thus much remains to be improved in terms of carrying out a uniform coating process.
In addition, the magnetic carrier core is mixed with the resin composition particles using a separate apparatus from the apparatus used for the coating process, and the requirement for a separate mixing apparatus is inconvenient.
The following method is disclosed in Japanese Patent Application Laid-open No. 2010-128393.
A mixture is first obtained by mixing a magnetic carrier core and resin composition particles at a magnetic carrier core:resin composition particle weight ratio=97:3.
This mixture is then introduced into a Spartan Ryuzer (Dalton Co., Ltd.) and is stirred (total of 90 minutes at a peripheral velocity of 18.5 m/second). The apparatus temperature is raised as stirring progresses, and, after the temperature has reached 80° C., stirring is carried out for 60 minutes while maintaining the temperature.
This is followed by introduction into a circulating hot air current-type oven (SPHH, ESPEC Corp.) and heating over 1 hour at 200° C. to obtain the magnetic carrier by curing the resin coat layer on the magnetic carrier core surface.
However, residual resin composition particles may be produced when the resin composition particles are to be coated in large amounts on the magnetic carrier core, and thus much remains to be improved in terms of carrying out a uniform coating process.
In addition, the resin composition particles on the magnetic carrier core surface are cured in this method using a separate apparatus from the apparatus used for the coating process, and the requirement for a separate apparatus for curing is inconvenient.
In contrast to these methods that carry out the dry coating process based on heat, methods have been introduced that carry out the dry coating process based on mechanical impact force.
For example, a method is disclosed in Japanese Patent Application Laid-open No. S63 (1988)-235959 in which the magnetic carrier core surface is coated by resin composition particles having a particle diameter of not more than one-tenth that of the magnetic carrier core using a surface modification process apparatus that has a rotor and a stator.
Using resin composition particles that have a particle diameter of not more than one-tenth that of the magnetic carrier core, this method creates a single-layer coat layer of the resin composition particles on the magnetic carrier core surface and carries out coating with this by mechanical impact force.
However, when resin composition particles having a particle diameter that is more than one-tenth that of the magnetic carrier core are used in this method, these resin composition particles must be dispersed using an apparatus separate from the apparatus used for coating the magnetic carrier core surface.
When a dispersing apparatus is not used, resin composition particles then remain free and the favorable execution of coating of the resin composition particles on the magnetic carrier core surface is impaired.
In addition, even when the resin composition particles are attached to the magnetic carrier core surface using an apparatus different from the coating apparatus, excess resin composition particles end up being present in a free state when the resin composition particles are added in an amount too large for attachment and the execution of a uniform coating is then problematic.
Below, excess resin composition particles are referred to as residual resin composition particles.
Accordingly, there are limitations in this method on the amount of coating by the resin composition particles, and this may ultimately impair the ability to control the amount of toner charge and inhibit charge injection from the magnetic carrier to the photosensitive member.
In contrast to the preceding, in order to raise the amount of coating by the resin composition particles, a method has been disclosed in Japanese Patent No. 2811079 that uses a high-speed stirrer/mixer and that intermittently feeds the resin composition particles divided into at least two additions.
However, with this method also, residual resin composition particles that do not participate in coating are produced and the coverage ratio changes with each production of the magnetic carrier and magnetic carrier-to-magnetic carrier property variations are produced; as a consequence, it may not be possible to obtain a consistent magnetic carrier on an extended basis.
A processing apparatus is provided in Japanese Patent Application Laid-open No. 2011-2686 as another composite-forming processing apparatus that uses a mechanical impact force.
This processing apparatus, while exploiting the advantages of a rotating blade-type apparatus, can achieve an excellent coating of the magnetic carrier by raising the stirring effect by applying, to the processed material of the magnetic carrier core and resin composition particles, a force with a heretofore unavailable strength.
Furthermore, by carrying out coating with the resin composition particles a plurality of times, a substantial mitigation with regard to the smoothness of the surface and the residual resin composition particles can be achieved.
However, while the uniformity of the magnetic carrier surface is improved, variation is still present in terms of bringing about a uniform resin density within the coating resin layer and some nonuniformity is produced in how the coat layer is shaved off during durability testing: variation in the amount of toner charge then appears in the latter half of durability testing and fogging is produced in some cases. As a consequence, substantial improvement is still required.
A method of producing a highly durable two-component developer is provided in Japanese Patent Application Laid-open No. 2012-8368.
In this method, a porous ferrite and resin particles are mixed at a temperature below the glass-transition temperature of the resin particles to produce a carrier intermediate in which the resin particles are attached to the surface and in the pores of the porous ferrite. Film formation is then performed on the carrier intermediate at a temperature greater than or equal to the glass-transition temperature of the resin particles. As a result, a low specific gravity magnetic carrier with a uniform surface is obtained without requiring resin in the interior of the magnetic carrier and a highly durable two-component developer is thereby obtained.
However, the production of the carrier intermediate involves just the attachment of the resin particles in the neighborhood of the surface of the porous ferrite core, and film formation simply by stirring at a temperature greater than or equal to the glass-transition temperature when the resin coat layer is formed can result in the presence in a more or less scattered manner of gaps within the resin coat layer and thus in the appearance of scatter in the local resistance. As a result, depending on the resistance of the porous ferrite core, it may not be possible to inhibit charge injection from the magnetic carrier into the photosensitive member and a blank dot image may be produced. In addition, some bias may occur in how the coat layer is shaved off during durability testing and scatter in the amount of toner charge may be produced and fogging may be generated.
Thus, as has been described in the preceding, much remains to be improved in order—in the coating of resin composition particles on the surface of magnetic carrier core by a dry coating process—to uniformly coat the magnetic carrier core surface and achieve uniform coating even in the interior of the coat layer.